A newer trend in automotive heating, ventilating air conditioning (HVAC) systems is the use of flexible film dampers to replace conventional flapper door type dampers. Typically, the volume and direction of forced airflow through the evaporator and heater housing is controlled by swinging doors or flappers. Several doors swing back and forth the change the proportion of air that flows through one heat exchanger or the other, and to direct its outlet from one vent opening to another. An inherent drawback of this kind of control, apart from the volume taken up by the swinging arc, is the non linearity of its operation. That is, the door tends to open or close a flow path or vent window completely or not at all, but is very difficult to apportion accurately.
An alternative with potentially much better linearly, which is also more space effective, is a flexible film valve in which a roll of film is rolled back and forth, almost like a window shade, to block or unblock various openings to any degree desired. Existing film valve drives are expensive and complex, however. Two shafts are used with known designs, one to wind up an end of the belt and another to wind it off. Only one shaft may be powered, with the other shaft following along passively, with a return spring or other device used to maintain belt tension.
Other designs drive both shafts actively, although one shaft may be powered indirectly off of the other. Some designs incorporating two driven shafts are quite complex. One example can be seen in U.S. Pat. No. 5243,380. There, each roller is paired with a co axial, co rotating conical pulley located at one end, which is oppositely oriented relative to the conical pulley on the other roller. One roller and pulley pair is directly motor driven, while the other is indirectly driven by the first, either through the film or the wire. In one direction, the film is in tension and serves as the opposite roller driving means, while the wire is slack, and vice versa. Within each roller-pulley pair, as the belt winds up more thickly, the wire moves to a smaller pulley radius, and vice versa. Since the conical pulleys are oppositely directed, as the effective radius of one is growing, the other is contracting, and vice versa. The belt winds in the opposite direction of the wire, so that the winding up roller with its thickening wound layer of belt can rotate more slowly as the winding off roller with its thinning wound layer of belt compensates by rotating more quickly. While the mechanism does work with only one motor, it is somewhat complex, with its extra wires and pulleys. Moreover, the mechanism is very sensitive to misadjustment. Unless the counter rotating rollers and pulleys are timed and registered to one another precisely, the wire will not finish winding up simultaneously with the belt winding off, and vice versa. Repair and adjustment within the small, confined spaces available is very difficult.